The Birth of a New Physics, a book by I. Bernard Cohen

Recently the local DC Objectivist Salon (DCOS) had a meeting to discuss the book The Birth of a New Physics, by I. Bernard Cohen (originally published in 1960, then revised and updated by the author in 1985). Here is a brief report on the book.

The late professor I. Bernard Cohen was one of the first Americans to get a PhD in the History of Science, and he had an illustrious career in that field, focusing on the Scientific Revolution, but also writing about subjects as diverse as the American founding fathers and the history of computing. Having read chapters from a range of his books, I can attest that he is generally a good writer with a great respect for human achievement.

The Birth of a New Physics is a short book (184 pages) which attempts to present the essential scientific developments leading to the birth of classical physics, that is, the mechanical physics taught in basic high-school and college classes today. It does not present the wider historical context of these developments; but instead it focuses the reader's attention on the chain of basic discoveries.

The story begins with "Aristotle's Physics of Common Sense," which was, with minor changes, the dominant view of physics for almost two millennia. Beneath the celestial spheres, the motions of objects can be divided, in this view, into "natural" and "violent" motion. The center of the earth is the center of the cosmos and the natural place sought by all heavy objects. If I drop a stone, it falls to the ground. This is its natural motion. It is seeking its natural place, which is as close to the center of the earth as it can get. Motion in any other direction can only take place when an external force is applied; this is “violent” motion. If I push something like a cart, it rolls, and when I stop pushing, it soon comes to a stop. In this view, force causes motion; when force is withdrawn, the motion soon stops.

This geocentric view of Aristotle and Ptolemy was not doubted until after the work of the Polish canon Nicholas Copernicus, whose book Revolutions of the Heavenly Spheres was published in 1543. Most people are not aware that the Copernican system was basically as complicated (and as accurate) as that of Ptolemy. The Copernican system was not at all convincing because Copernicus did not have a good answer to the major objection: When you throw an object straight up, why does it come straight down, if the ground is in motion?

This question was not fully answered until the work of Galileo, and two chapters present Galileo’s discoveries with the telescope and his brilliant experiments with the inclined plane. This led him to the basic law of inertia.

The following chapter presents the ideas of the “tortured mystic” Johannes Kepler, who discovered the elliptical shape of planets’ orbits. Kepler is somewhat unique in this story. He “stumbled onto his great discoveries in a weird groping that has led one of his biographers to call him a ‘sleepwalker.’ Trying to prove one thing, he discovered another, and in his calculations he made some major errors that cancelled each other out.” Although Kepler is most famous now for his three laws of planetary motion, he was most proud of his "discovery" that the nested Platonic solids explain the number of the planets and their distances from the sun.

The final and climactic chapter of the book presents Isaac Newton's grand synthesis, which was based on a masterful integration of the discoveries of Galileo and Kepler. Newton’s synthesis is summed up by his three laws of motion and his law of universal gravitation.

The Birth of a new Physics is flawed with respect to organization, and to a lesser extent, clarity. Tacked onto the end of the book are sixteen “supplements” which read like a random list of extra comments. Although some of these are quite interesting, most of them should have been folded into the main text or left out entirely. In several places Cohen seems to count on the reader knowing a definition which he doesn’t present until several pages later. In other places Cohen adds needless digressions which interrupt the main flow of ideas.

However the book largely succeeds in its mission: to concisely present the essential chain of discoveries leading from the physics of Aristotle to that of Newton. This is actually the only book I know of which attempts to do this. Moreover, unlike many of today’s academic historians of science, Cohen does not have a cynical view of science. Cohen has a genuine and deep respect for scientific achievement, and that makes this book a pleasure to read.